1. Technical Field of the Invention
The present invention relates to telecommunication systems and, in particular, to measurement techniques for achieving diversity and inter-frequency mobile assisted handoff (MAHO).
2. Description of Related Art
Mobile wireless communication is becoming increasingly important for safety, convenience, and efficiency. One prominent mobile communication option is cellular communication. Cellular phones, for instance, can be found in cars, briefcases, purses, and even pockets. Cellular phones, like most mobile communication options, rely on the transmission of electromagnetic radiation from one point to another.
In general, a cellular system is composed of many cells, each with a base station antenna for receiving transmissions. From the base station, the cellular system has interfaces for routing a call through or to the land-based, or terrestrial, telephone network, often referred to as the public switched telephone network (PSTN). The base stations form one half of the cellular system. Cell phones, called mobile stations, mobile terminals, or merely terminals, form the second half of the cellular system. In short then, electromagnetic radiation transmissions between terminals and base stations are an essential component of cellular systems, and such transmissions must be optimized by the cellular system to maximize cellular phone service, quality, and availability.
Properly operating cellular systems also requires significant planning, organization, and management. For instance, there must be a sufficient number of base stations to ensure a minimum level of service. Also, base stations must communicate with one another sufficiently to be able to coordinate a mobile terminal's transfer from one cell to another, termed handoff or handover. Furthermore, the portion of the electromagnetic spectrum that is allocated to a cellular system must be efficiently utilized.
Many different cellular phone system standards have been developed in response to these service, coordination, and efficiency requirements. Two examples of standards are the Global System for Mobile Communications (GSM) and the Advanced Mobile Phone Service (AMPS). Early standards were analog, but subsequent ones were digital. One standard, TIA/EIA/IS-95-A: Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System (IS-95) that was promulgated in May of 1995 by the Telecommunications Industry Association, combines analog and digital concepts to enable cellular phone users to access both analog and digital systems. With IS-95, a dual-mode terminal will interface with a digital system when possible, but the terminal can also connect to an analog base station as well.
When communicating via the transmission of electromagnetic radiation, diversity can be used to counteract signal fading, which occurs when a signal's strength decreases. A given radio signal will usually take several diverse paths from the transmitter to the receiver. As a result, the receiver actually has several versions of the same signal from which to choose for processing. Often, the different paths will not be fading simultaneously, so if the receiver can always be processing the version of the signal with the least fade at a given moment, then the overall transmission will be more reliably received and processed. Diversity, then, is a term applied to various techniques for creating and/or selecting the current optimum version of the signal.
Some examples of diversity schemes are space, polarization, angle, frequency, time, and path diversity. Path diversity was explained above as occurring when an original signal follows multiple paths. These multiple paths arise from the signal taking a direct path or any one of many reflective paths. As a second example, polarization diversity is noted. In polarization diversity, the output from one of two antennas is selected by choosing the antenna that is properly polarized with the strongest signal. It offers benefits because signals transmitted on orthogonal polarizations exhibit uncorrelated fading. While only two diversity branches are available, polarization diversity can be especially important for hand-held mobile terminal communications because the hand-held devices are held at various angles during a phone call.
Another benefit of diversity results from the reduction in transmission power requirements. Transmission power requirements are reduced because selection by a receiver of the strongest signal from among diverse, uncorrelated signals enables a transmitter to reduce power. This power reduction improves interference tolerance, which results in an ability to support additional users within a given cell. In short, the entire range of diversity techniques provides increased interference immunity, which is especially important for interference-limited systems such as IS-95.
Another technique to improve cellular phone service and quality that some cellular system standards use is inter-frequency mobile assisted handoff (MAHO). Inter-frequency MAHO improves the process a mobile terminal undertakes when moving between microcells and macrocells. With inter-frequency MAHO, the mobile terminal provides assistance by making measurements on frequencies that differ from the frequency currently being used. Inter-frequency MAHO is especially important when dealing with hierarchical cell structures. In cellular systems with hierarchical cell structures, handoffs between microcells and macrocells can be more effectively performed by using the measurements provided by the mobile terminal. Consequently, the ability to make measurements on other frequencies is highly desirable when a mobile terminal is in motion.
Different multiple access methods are used by the different cellular system standards. Each method attempts, among other things, to efficiently utilize the limited resources of base stations and electromagnetic spectrum allocation. Other goals of access methods include maximizing capacity and service quality while minimizing implementation costs.
One access method is Frequency Division Multiple Access (FDMA). It divides the allocated spectrum into nonoverlapping segments and allots to the terminal of each cellular user a frequency segment on which to transmit.
Another access method is Time Division Multiple Access (TDMA). It permits each terminal to use a given frequency for a limited time. Then a different terminal is permitted to use the same frequency. Shortly, the original terminal is again permitted a slice of time in which to use the frequency. In this technique, several cellular users share the same frequency or frequency segment over time in a nonoverlapping, round-robin fashion.
Yet another access method is Code Division Multiple Access (CDMA). In this spread-spectrum technique, each terminal is permitted to use the entire spectrum allocated for a channel in a given cell at all times. The base station differentiates one terminal's signal from that of another by detecting a digital code embedded in the transmission of each terminal's signal. This code arises because the transmitted signals have very small cross-correlation. Therefore, correlators can be used to extract individual signals from a mixture of signals even though they are transmitted simultaneously and in the same frequency band. IS-95's digital mode, for example, utilizes CDMA.
As explained above, diversity is important to efficient cellular communication, regardless of the cellular system standard or access method employed. To achieve diversity, the diverse signals must be combined in some preferably optimum way. In short, the best signal or the best combination of signals should be extracted. One such combination technique is selection combining, or specifically pre-detection selection diversity (or merely pre-selection diversity). In selection diversity, the receiver attempts to choose the diversity branch with the highest carrier to noise ratio (C/N). More specifically, in pre-detection selection diversity, the selection of the antenna is made before reception of the desired signal.
Pre-detection selection combining has typically been proposed for TDMA systems, in which a mobile terminal does not need to receive continuously. Hence, the TDMA mobile terminal can simply sample the signal on both antennas during non-allotted time frames. Then it can make an appropriate selection before its allotted receive time frame occurs. In TDMA systems, pre-detection selection combining enables the use of only one receive chain when implementing selection combining.
In contradistinction to TDMA systems, CDMA systems in general and IS-95 systems in particular normally require a mobile terminal to receive signals from a base station continuously. Hence, the CDMA mobile terminal (with only one receive chain) cannot simply sample the signal on the second antenna during non-allotted time frames because IS-95 does not specify any non-allotted time frames. Similarly, because the signal is being received continuously, the CDMA mobile terminal cannot switch to another frequency for the purpose of making measurements for inter-frequency MAHO. Thus, heretofore, continuous reception cellular system standards hindered the implementation of selection diversity and inter-frequency MAHO and thus the enjoyment of the accompanying benefits.
Therefore, one object of the present invention is to enable diversity and inter-frequency MAHO in continuous reception systems, such as IS-95.
Another object of the present invention is to provide measurement techniques for achieving diversity and inter-frequency MAHO in continuous reception systems, such as IS-95.
Another object of the present invention is to provide a method to perform pre-detection selection diversity combining in continuous reception systems, such as IS-95.
Yet another object of the present invention is to provide a simple method for implementing a diversity receiver for the downlink of a continuous reception system, such as IS-95.
Yet another object of the present invention is to provide a method for selecting between two antennas for phones in continuous reception systems, such as IS-95, based on signal strength measurements.
Yet another object of the present invention is to implement the above methods and apparatuses while minimizing bit error rates.